CTRL works fine, but the ALT does not produce a guideline. The courser will move to the vertical position as if to place a guideline, but no guide line will be placed.
I'm also pretty sure that it did work at one point. Now nothing...
I even tried my old keyboard, but it's the same on that too.

Wallace: I just came across a note I made regarding that windows update.
The thumbnail view on internet explorer and gimp preview stopped functioning. Did not bother me as Faststone viewer runs permanently on my desktop.
I have no interest in pursuing a remedy as msw8.1 has run me out of headache pills.
Long live Linux. Or bring back XP

I don't get it?
I still have my old machine running xp and Gimp 2.6.11.
I was going to install a copy of Gimp 2.6.11 on my new computer, but haven't done it yet.

Sometimes problems such as this are caused by the ALT key being intercepted by the GUI. You might try using ALT+SHIFT click instead of ALT+click. You might also check your accessibility settings.

I know I'm doing it right, because that's how it's suppose to work. CTRL will add a horizontal guideline and ALT adds one vertically. That's how it has always worked for me and that is what the Gimp documentation states.

I'm not sure what you mean by checking my accessibility settings. Do you mean my Windows accessibility settings? Because I'm not aware of this in Gimp.
BTW, CTRL+ALT doesn't work either when using the measuring tool.

It is not a matter of you doing anything wrong, it is a matter of your GUI (hypothetically) intercepting your ALT+click before it ever gets to GIMP. For example (hypothetical), if Windows assigned ALT+click to open the Properties dialog for the object under the cursor, then GIMP would never see an ALT+click. However, if you hold down ALT+SHIFT while clicking, Windows would ignore it, and GIMP would see if the ALT key is being held down on the click (GIMP will ignore the SHIFT key because nothing has been assigned to ALT+SHIFT+click) and will execute the appropriate command.

by saulgoode
I've tested every Windows 7 keyboard shortcut that includes the use of the ALT key and they all function as they should.
I've also tested some of the ALT key functions that I know of within other programs and all I've tested thus far work properly.
So I'm now thinking it has something to do with Gimp.

Doing ALT+SHIFT+right click does nothing and there isn't anything within windows that I'm aware of that uses the ALT+Right Click.
The ALT key does not seem to have any function in Gimp. Although when I try to place a guideline vertically at the end point of a measurement, the courser does move to the vertical position, but no guide is placed.
I feel like I've been repeating myself.

I found out what was causing this issue. Now all I need to do is install the Script-Fu Interface Scrolling Modification, because the issue was being caused by AltDrag.

For me on Windows 7 GIMP 2.8.10 it works

I create the measurement, then press and hold the ALT key, move the cursor over the required end point and the cursor changes to the vertical line and arrow, then left-click and the guide is added.

You could check your new keyboard is generating ALT key codes. I've just tried downloading and running KeyCodes Display: http://delphiforfun.org/programs/utilities/KeyCodes.htm and that shows I get when I press the left ALT key

Kevin

by you

Thanks Paynekj, but I found what was causing the problem.

I find the same as Ofnuts with PCLinuxOS KDE and Gimp 2.8.10
alt on its own does not work
ctrl + alt places both horizontal and vertical guides
but strangely
left win + alt places a vertical guide (I know a linux machine and the win key). Why it should be mapped this way, I do not know. Looking at the linux keyboard options, none of the win/alt options are enabled.

After all I've been through yesterday I'd like to forget about this stupid ALT key thing. I'm just relived that I found what was wrong, and it was right under my nose all the time too. Which doesn't make the fact that I found the solution to the issue any better.

So the problem was that this add-on program, Altdrag, was intercepting ALT+click and preventing GIMP from receiving it? If so, then I would note that the program also intercepts ALT+SHIFT+click and this is why that combination failed to work.

In Linux (version in signature) I'm unable to use ALT click on an end point to create a vertical guide, although I seem to remember in the past I could. The mouse pointer shows the correct icon but without success. Shift + ALT works. In my case saulgoode might have identified why.

Yes that's what was happening alright. Funny thing about this is, that I picked this AltDrag thing up to deal with some of the large dialog windows in Gimp. I can still use the AltDrag, but not while I'm working with Gimp.
I wonder if it would have any effect on Photoshop like it did on Gimp.


by Odin...
Yes saulgoode was right.
If he hadn't mentioned this I think I'd still be looking to solve the issue.

You might be able to find a different ALT+[mod] that will work for the GIMP guides. In theory, the AltDrag program should pass through to the application any ALT+ operations that it doesn't recognize, so perhaps ALT+WINKEY, ALT+WINKEY+SHIFT, or somesuch would work.

You might also want to contact the author of the program and describe your experience. It is a fairly common paradigm under GNU/Linux that if an ALT+ combination doesn't work (because the window manager is already using it), then the ALT+SHIFT combination should provide a work-a-round. It might be better if the AltDrag program used something other than ALT+SHIFT+drag for window snapping.

So far, Gimp is the only program that this has had any effect on. The program is really a script that doesn't require installation and runs in the background, I can turn it on or off at will. I also can remember being able to use the ALT+click and CTRL+ALT for placing guidelines in Gimp. I've been using AltDran for a wile now. It's only as recent as this week that this has been happening.
Contacting the author is a good point that I hadn't thought about and will do so.
It would be nice if AltDrag had an options for changing it's key assignment. As it stands, I use to have AltDrag running continually in the background. Now I only run it when it's needed.
I really don't understand why windows OS has this kind of constraint anyway, and the fact that I have to resort to using this AltDrag thing as a work-around is ridiculous at best.
Thanks Saulgoode, your input has really been instrumental in helping me solve this annoying issue.